In certain railroad operations, such as, in the separation and sorting of incoming trains in a classification yard, it is necessary to detect the presence of railway cars at various locations along their route of travel to the selected class tracks. For example, in an automatic humping operation, it is common practice to employ frictional braking apparatus or car retarders for controlling the speed of the freely-moving vehicles in order to ensure that the vehicles safely reach their destination and/or properly couple with other vehicles standing in the appropriate class track. In order to apply the appropriate braking force to the sides of the passing wheels, it is indispensable to detect and determine when a railway vehicle is within the confines of the car retarding assembly. While numerous types of wheel detectors have operated satisfactorily in certain installations, it has been found that these previous wheel detectors are not suitable for car retarder applications. For example, many of the prior art detectors are relatively large and bulky, and therefore, they do not lend themselves to the confined quarters and limited amount of space which is available in a retarder installation. That is, the use of conventional wheel detectors in a frictional-gripping type car retarder is not plausible since the clearance requirements cannot be met so that proper operation of the retarder is not possible. In addition, the previous wheel detectors were susceptible to damage and even destruction by automatic ballast cleaning and tamping equipment as well as by dragging equipment. Further, magnetic types of car detectors are adversely effected by the great amount of iron and steel that make up the massive retarders. In one case, it was proposed to drill holes in the top of the head of the running rails and to insert and cement magnetic coils therein to detect the passing wheels of moving railway vehicles. However, this proposal was not acceptable in that the drilled hole weakened the rails and could lead to fatigue and cracks and eventual failure of the running rails. In addition, the extreme vibrations, pounding, as well as the thermal expansion and contraction of the rail adversely effects the operation of conventional types of magnetic, mechanical and light-responsive types of wheel detectors in a railway car retarder installation.